I wasn't either - but when I found out how hard it was to come by a really old 15-30, when I found one I had to have it. Actually, I was after the cabinet and "assumed" it was a 115 as the seller didn't know, and when I got there I was surprised to turn it over and see that it was a 15-30 after all. So it got added to my collection.
I'm pretty sure I'm not interested in any more old Singers - got tempted by a Pheasant decal 27, but I sat on my hands until it was sold.
I've got four machines on my "want" list, and if things go well I'll cross another off later tonight.
BTW - after getting the 319 and checking the bobbins, the one in the 319 is the same as the one in the 115 (which is not the same as the 15).